The invention relates to the beneficiation of ores and the like, and, more particularly, to a process for beneficiating and utilizing the fine particulate fraction of phosphate-containing ore.
Many important metals and chemicals are obtained in commercial quantities by mining ores that contain fairly low concentrations of the desired values in a chemically combined form, and then processing and refining the ores to yield the desired final product having a high concentration of relatively high purity material. To be efficient, most refining operations that process the chemically combined form of the mineral values to the desired product require as a starting material a modified ore that has a higher concentration of mineral values than that found in the ore when mined. Additionally, some impurities found in the as-mined ore can seriously interfere with the refining operation. In a treatment termed beneficiation utilized prior to refining, the as-mined ore is processed to increase its mineral value content and reduce particularly harmful impurities to a tolerable level in preparation for refining operations.
As a first step, the as-mined ore is mixed with water to form a slurry having 25-35 percent solids content, and comminuted, or crushed, to produce ore particles that contain the desired mineral values. During comminution, mineral particles having a wide range of sizes are produced, ranging from less than 20 micrometers to about 0.010 inch in size, in a typical sizing operation.
It is usually desired that the ore particles be rather small in size, so that the mineral values locked within the ore particles are at or near the surface and can be liberated in subsequent processing. In some operations the particles must be small in size to accommodate the requirements of particular processing machinery. For example, when the slurry is to be pumped through a slurry pipeline, the particles should be relatively small. To achieve small sizes, the slurry can be comminuted and sized and the portion containing coarse particles recycled through the comminution machinery to crush those particles that were not sufficiently reduced in a prior pass. The result of the recirculation is that the largest sizes of particles are reduced to acceptably small sizes, but also that a relatively large fraction of very fine ore particles are produced. Mixed in with the ore particles are fine pieces of various undesirable constituents that are unavoidably mined with the minerals and contain a high fraction of impurities, such as clay, silicates, carbonates, and the like, collectively termed "gangue".
The gangue particles often closely associate with the finest ore particles, which are of a size closer to the size of the gangue pieces than are the larger ore particles. The resulting slurry of fine particles and gangue is of a mud-like consistency, and is often termed a "slime".
In many beneficiation operations, the slime is separated from the slurry containing the coarse particles, and then discarded to piles or pits. The fine particles in the slime contain a substantial portion of the mineral values of the mined ore, so that the standard practice of discarding the slime results in the loss of a significant portion of the mineral values. Even with a major portion of the impurity content removed with the gangue in the slime, the remaining coarse ore particle slurry may have too high an impurity content for subsequent refining. Further beneficiation steps are therefore necessary before the ore can be refined.
An example illustrates the problems arising in beneficiating some ores. Certain Western phosphate ores with phosphate values of 25 to 35 percent contain various metallic (or clay) silicates that include aluminum (and usually other ions such as sodium, potassium, and calcium). Aluminum is present in such silicates in an amount of 1.5 to 2 percent of the ore, expressed as aluminum oxide (the conventional mode of expression in the industry). The high aluminum content can be detrimental to subsequent manufacture of phosphoric acid.
The ores are comminuted, producing a slurried mixture of coarse ore particles having sizes up to about 0.010 inch, fine ore particles having sizes less than about 20 micrometers, and a gangue having a high content of silicate clay. The fine ore particles and gangue mix to form a slime. The aluminum impurity reports unevenly to the fractions with the portion containing coarse particles having about 1 percent aluminum expressed as aluminum oxide, and the slime portion containing fine particles and gangue having about 2 to 3 percent aluminum expressed as aluminum oxide. If the slime is permitted to remain in the slurry of coarse ore particles, the high aluminum content can interfere with refining of the phosphate values from the ore in subsequent processing.
In the past, it has been common practice to remove the high-aluminum slime from the ore slurry, as in a hydrocyclone. The slime, containing fine ore particles, was discarded, and the remaining material had a reduced content of the harmful aluminum impurity. However, a significant portion of the phosphate mineral values can be lost in this way. There have been attempts to extract the mineral values from the slimes, but these have been unsuccessful, with the result that discarding of the phosphate values in the slime has become standard practice. No practical method of beneficiating the ore is known, so that the phosphate values in the slime can be recovered and the aluminum content of the ore reduced to an acceptably low level for further processing.
Accordingly, there exists a need for a method of beneficiating ores to recover an increased portion of the mineral values therein, and simultaneously reduce the content of those impurity elements whose presence interferes with subsequent refining. Such a method must be economically viable and not preclude other processing operations. The present invention fulfills this need, and further provides related advantages.